In the sampling mixer, signals that underwent a digital modulation are sampled by a sampling circuit, and thus a filter effect can be obtained by a switched capacitor built in the sampling circuit (for example, Patent Literature 1).
FIG. 6 is a circuit diagram of a sampling mixer 600 set forth in Patent Literature 1, and FIG. 7 is a timing chart of control signals of the sampling mixer 600.
In FIG. 6, the sampling mixer 600 includes a TA (transconductance amplifier) 1 for converting a received radio frequency (RF) signal into an RF current iRF, an in-phase mixer section 2 for sampling the RF current iRF generated by TA 1, an opposite-phase mixer section 3 combined with the in-phase mixer section 2, and a DCU (digital control unit) 4 for generating control signals for the in-phase mixer section 2 and the opposite-phase mixer section 3.
The in-phase mixer section 2 includes a sampling switch 5, and a Ch (history capacitor) 6 for temporally integrating the signals sampled by the sampling switch 5 successively. Also, the in-phase mixer section 2 includes a plurality of Crs (rotate capacitors) 7 to 14 for repeating the integration and the discharge of the signals sampled by the sampling switch 5, and a Cb (buffer capacitor) 15 for buffering the signals discharged from respective Crs 7 to 14.
The in-phase mixer section 2 includes a damp switch 16 for discharging the signals held by respective Crs 7 to 14 to the Cb 15, a reset switch 17 for resetting the signals held on respective Crs 7 to 14 after the signal discharge, and a plurality of integrate switches 18 to 25 for connecting the Ch 6 sequentially to respective Crs 7 to 14.
The in-phase mixer section 2 includes a plurality of discharge switches 26 to 33 for connecting sequentially respective Crs 7 to 14 to the Cb 15, and a feedback switch 34 for controlling the input of a feedback signal from a DA (Digital-Analog) converter to the sampling mixer 600 side.
Next, while taking an operation of the in-phase mixer section 2 as an example, an operation of the sampling mixer 600 will be explained hereunder.
First, the RF current iRF is sampled by the sampling switch 5, and is discretized on a time axis to give a discrete signal.
The discrete signal is integrated sequentially by the Ch 6 and respective Crs 7 to 14 based on a SV0 signal to a SV7 signal, and the filtering and the decimation are applied to the resultant signals.
Thus, the effect of an 8-tap FIR (Finite Impulse Response) filter can be attained. A sampling rate at this time is decimated to ⅛. This is because a moving average is applied to the signals held in the eight integrate switches 18 to 25. Such filter is called a first-stage FIR filter. A transfer function of the first-stage FIR filter is expressed by following Expression.
                    [                  Formula          ⁢                                          ⁢          1                ]                                                                      H                      FIR            ⁢                                                  ⁢            1                          =                              1            -                          z                              -                8                                                          1            -                          z                              -                1                                                                        (                  Expression          ⁢                                          ⁢          1                )            
Also, since the Ch 6 connected sequentially to respective Crs 7 to 14 holds an output potential, the effect of an IIR (Infinite Impulse Response) filter can be attained. Such filter is called a first-stage IIR filter. A transfer function of the first-stage IIR filter is expressed by following Expression. Here, a capacitance value of the Ch6 is assumed as Ch, and capacitance values of respective Crs 7 to 14 are assumed as Cr, respectively.
                    [                  Formula          ⁢                                          ⁢          2                ]                                                                      H                      HR            ⁢                                                  ⁢            1                          =                  1                      Ch            +            Cr            -                          Chz                              -                1                                                                        (                  Expression          ⁢                                          ⁢          2                )            
Also, when an SBZ signal is input into gates of the discharge switches 26 to 29, all discharge switches 26 to 29 are turned ON in a high-level period of the SAZ signal. Then, the discrete signals integrated by respective Crs 7 to 10 are discharged simultaneously to the Cb 15 via the discharge switches 26 to 29 that are in an ON state.
After this discharge, then a D signal goes to a low level. Thus, the damp switch 16 is turned OFF, and the Cb 15 is disconnected from respective Crs 7 to 10. Then, an R signal goes to a high level. Thus, the reset switch 17 is turned ON, and the signals held in respective Cr 7 to 10 are reset.
Thus, the signals integrated by respective Crs 7 to 10 are discharged simultaneously to the Cb 15. Accordingly, the effect of a 4-tap FIR filter can be attained. A sampling rate at this time is decimated to ¼. This is because the signals integrated by four Cr 7 to 10 are moving-averaged to the Cb 15.
Also, the signals integrated by respective Crs 11 to 14 function similarly to the case of respective Crs 7 to 10 using an SAZ signal instead of the SBZ signal. Such filter is called a second-stage FIR filter. A transfer function of the second-stage FIR filter is expressed by following Expression.
                    [                  Formula          ⁢                                          ⁢          3                ]                                                                      H                      FIR            ⁢                                                  ⁢            2                          =                              1            4                    ⁢                                    1              -                              z                                  -                  4                                                                    1              -                              z                                  -                  1                                                                                        (                  Expression          ⁢                                          ⁢          3                )            
Also, four Crs are connected to the Cb 15 in a group of four Crs 7 to 10 or four Crs 11 to 14. Accordingly, the effect of the IIR filter can be attained. Such filter is called a second-stage IIR filter. A transfer function of the second-stage IIR filter is expressed by following Expression. Here, a capacitance value of the Cb 15 is assumed.
                    [                  Formula          ⁢                                          ⁢          4                ]                                                                      H                      IIR            ⁢                                                  ⁢            2                          =                              4            ⁢                                                  ⁢            Cr                                              4              ⁢                                                          ⁢              Cr                        +            Cb            -                          Cbz                              -                1                                                                        (                  Expression          ⁢                                          ⁢          4                )            
In this case, the opposite-phase mixer section 3 operates substantially similarly to the in-phase mixer section 2, except that the sampling is delayed by a ½ period rather than the in-phase mixer section 2.
As the sampling mixer 600 is constructed in this manner, an output signal of the sampling mixer 600 is given as the signal that passed through the first-stage FIR filter, the first-stage IIR filter, the second-stage FIR filter, and the second-stage IIR filter. A transfer function of the overall filter is given by a following Expression using the current converting expressions based on Expression (1), Expression (2), Expression (3), and Expression (4). Here, a transconductance of the TA1 is assumed as gm, and a frequency of the RF signal is assumed as fRF.
                    [                  Formula          ⁢                                          ⁢          5                ]                                                                                                H              =                            ⁢                                                gm                                      π                    ⁢                                                                                  ⁢                                          f                      RF                                                                      ⁢                                  H                                      FIR                    ⁢                                                                                  ⁢                    1                                                  ⁢                                  H                                      IIR                    ⁢                                                                                  ⁢                    1                                                  ⁢                                  H                                      FIR                    ⁢                                                                                  ⁢                    2                                                  ⁢                                  H                                      IIR                    ⁢                                                                                  ⁢                    2                                                                                                                          =                            ⁢                                                gm                                      π                    ⁢                                                                                  ⁢                                          f                      RF                                                                      ⁢                                                      1                    -                                          z                                              -                        8                                                                                                  1                    -                                          z                                              -                        1                                                                                            ⁢                                  1                                                            (                                                                        C                          H                                                +                                                  C                          R                                                                    )                                        -                                                                  C                        H                                            ⁢                                              z                                                  -                          8                                                                                                                    ⁢                                  1                  4                                ⁢                                                      1                    -                                          z                                              -                        32                                                                                                  1                    -                                          z                                              -                        8                                                                                                                                                                                  ⁢                                                4                  ⁢                                                                          ⁢                                      C                    R                                                                                        (                                                                  4                        ⁢                                                                                                  ⁢                                                  C                          R                                                                    +                                              C                        B                                                              )                                    -                                                            C                      B                                        ⁢                                          z                                              -                        32                                                                                                                                                    (                  Expression          ⁢                                          ⁢          5                )            
Next, the filter characteristics containing various filters mentioned above will be explained with reference to FIG. 8 hereunder. Here, it is assumed that a frequency of the LO signal is assumed as 2.4 GHz, the Ch 6 is assumed as 15 pF, the Cr 7 to 14 are assumed as 0.5 pF respectively, the Cb 15 is assumed as 15 pF, and the transconductance of the TA1 is assumed as 7.5 mS.
The characteristic of the first-stage FIR filter is shown in FIG. 8A, and the characteristic of the first-stage IIR filter is shown in FIG. 8B. Also, the characteristic of the second-stage FIR filter is shown in FIG. 8C, and the characteristic of the second-stage IIR filter is shown in FIG. 8D. Also, the characteristic of the overall sampling mixer 600 is shown in FIG. 8E.
In the sampling mixer 600 constructed in this manner, the signal that passed through four filters, i.e., the first-stage FIR filter, the first-stage IIR filter, the second-stage FIR filter, and the second-stage IIR filter, is output to the AD converter.    Patent Literature 1: JP-A-2004-289793 (pages 6 to 9, FIG. 3a, FIG. 3b, FIG. 4)